Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reactant or reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane ("PEM") as the electrolyte, the hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is the aforesaid proton exchange membrane ("PEM") electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention.
Manufacture of fuel cells utilizing PEM electrolytes typically involves securing an appropriate first catalyst layer, such as a platinum alloy, between a first surface of the PEM and a first or anode porous substrate layer to form an anode electrode adjacent the first surface of the PEM, and securing a second catalyst layer between a second surface of the PEM opposed to the first surface and a second or cathode porous substrate layer to form a cathode electrode on the opposed second surface of the PEM. The anode catalyst, PEM, and cathode catalyst secured in such a manner are well-known in the art, and are frequently referred to as a "membrane electrode assembly", or "M.E.A.", and will be referred to herein as a membrane electrode assembly. In operation of PEM fuel cells, the membrane is saturated with water, and the anode electrode adjacent the membrane must remain wet. As hydrogen ions produced at the anode electrode transfer through the electrolyte, they drag water molecules in the form of hydronium ions with them from the anode to the cathode electrode or catalyst. Water also transfers back to the anode from the cathode by osmosis. Product water formed at the cathode electrode is removed from the cell by evaporation or entrainment into a gaseous stream of either the process oxidant or reducing fluid. In fuel cells containing porous reactant flow fields, as described in U.S. Pat. No. 4,769,297, owned by the assignee of all rights in the present invention, a portion of the water maybe alternatively removed as a liquid through the porous reactant flow field to a circulating cooling fluid.
While having important advantages, PEM cells are also known to have significant limitations especially related to liquid water transport to, through and away from the PEM, and related to simultaneous transport of gaseous reducing fluids and process oxidant fluids to and from the electrodes adjacent opposed surfaces of the PEM. The prior art includes many efforts to minimize the effect of those limitations. Use of such fuel cells to power a transportation vehicle gives rise to additional problems associated with water management, such as preventing the product water from freezing, and rapidly melting any frozen water during start up whenever the fuel-cell powered vehicle is operated in sub-freezing conditions. Known fuel cells typically utilize a coolant system supplying a flow of cooling fluid through the fuel cell to maintain the cell within an optimal temperature range. Where the cooling fluid is a solution including water it also must be kept from freezing. It is known to utilize an antifreeze solution such as ethylene glycol and water or propylene glycol and water as a cooling fluid in such coolant systems. However, such antifreeze solutions are known to be adsorbed by and poison the catalysts that form electrodes. Furthermore, those antifreeze solutions have low surface tensions which results in the solutions wetting any wetproofed support layers adjacent cell catalysts, thereby impeding diffusion of reactant fluids to the catalysts, which further decreases performance of the electrodes. Also, the vapor pressure of those antifreezes is too high, resulting in excessive loss rates of the antifreeze solutions through fuel cell exhaust streams or from steam produced in boilers of fuel processing components of fuel cell power plants. Therefore coolant systems of fuel cells that utilize an antifreeze solution are known to be sealed from the electrodes, so that the solution is not in direct fluid communication with the electrodes. Sealing the coolant system from direct fluid communication with the cell and hence with the product water formed at the cathode electrode results in decreased cell performance due to less efficient removal of the product water. Fuel cells with sealed coolant plates typically remove product water as an entrained liquid. This requires a tortuous serpentine flow path with a resultant high pressure drop. An example of such a cell is shown in U.S. Pat. No. 5,773,160. That type of cell is not suitable for operating at near ambient reactant pressures which is a preferred operating pressure for many fuel cell systems. The decreased performance of cells with sealed coolant plates requires that additional cells be used to satisfy the design power requirement. The additional cells combined with heavier coolers associated with sealed coolers results in an increase in weight and volume of a fuel cell power plant which is undesirable for a fuel cell used to power a vehicle.
Additionally, where a fuel cell powers a vehicle, the atmosphere serving as a process oxidant stream directed into contact with the cathode electrode will vary significantly in humidity. Consequently, it is known to undertake substantial efforts to humidify the process oxidant and reducing fluid streams in order to minimize water loss from the PEM electrolyte. Known efforts include recycling some of the product water from the cell, and/or directing some of the cooling fluid within the coolant system as a vapor into the process oxidant and/or reducing fluid streams entering the fuel cell. However, with known fuel cells, the humidity enhancing fluid must be free of any antifreeze solutions in order to prevent the antifreeze from poisoning the catalysts. Therefore, known fuel cells utilize sealed coolant systems that are isolated from humidification systems, or alternatively known humidification systems utilize complex, heavy and large purification or membrane barrier components to isolate from contact with the electrode catalysts any antifreeze solution within the cooling fluid or within the product water mixed with cooling fluid. Such efforts to isolate the antifreeze solution add to the cost, weight and volume of the fuel cell. Accordingly there is a need for a fuel cell that may be operated in sub-freezing conditions that does not require isolating an antifreeze cooling fluid from the cathode and anode electrodes within a sealed coolant system and that also minimizes free water within the system that may be frozen when the fuel cell is not operated.